Needle-free injectors (NFIs) operate by creating a high pressure jet of fluid/powder that penetrates the skin. Delivery is rapid (typically <0.5 s) which reduces apprehension while enhancing patient acceptance and ultimately compliance. In addition, NFIs have been shown to improve the efficacy of certain medications (Taylor et al. 1981; Jackson et al. 2001; Williams et al. 2000). Several needle-free injectors use springs or compressed inert gases to propel fluid through the skin and into the underlying tissue. This affords minimal control over the pressure applied to the drug during the time course of the injection, parameters shown to be integral to determining the depth and dispersion of drug delivered (Wendell et al. 2006; Shergold et al. 2006), and hence its absorption into the circulation. Others have incorporated some pressure pulse shaping by using variable orifice gas valves or fast/slow pyrotechnic charges. More recently, Stachowiak et al. (2009) have used piezoelectric actuators for dynamic control of delivery, accomplished at the expense of a limited piston stroke and volume of fluid delivered. An alternative approach to jet drug delivery is to store energy in electrical form and impose a time varying pressure profile (waveform) on the drug volume through the use of a monitored and servo-controlled electromechanical actuator such as a linear Lorentz-force actuator. Needle-free injectors may include sensors that can be used to control the actuator.
There are a great many multi-axis force sensors utilizing the measurement of strain to discern the magnitude and direction of an applied force. Most multi-axis force sensors however have assemblies that require a significant length available in the axial direction to be implemented, are difficult/costly to manufacture, have an unnecessarily redundant number of locations in which strain is measured, cannot have a drug ampoule of a needle-free injector mounted to them (and associated piston pass through them), result in significant displacements when loaded (of which a subsequent problem is inaccuracy in the injected volume or misalignment between the piston and the rest of the ampoule), have flexures that are limited to exactly twice the length of the sensing element to produce maximum signal which increases the overall radius required of the device, or have not been reduced to practice for measuring forces on the nozzle of a jet injector drug delivery ampoule assembly.